Phase locked loops utilize voltage controlled oscillators to maintain phase and frequency stability, wherein a control voltage resulting from a comparison between an output signal and a reference signal is used to control the phase and frequency of an oscillator. However it is common that rather than a voltage controlled oscillator, a current controlled oscillator is used, with a voltage to current converter feeding it, wherein the control voltage signal is applied to the voltage to current converter.
The input voltage to the converter is typically obtained from a charge pump and loop filter combination as described by F. Gardner in "Charge-Pump Phase Lock Loops", IEEE Transactions on Communications 28; 1849-58, November 1980. In a single V/I converter the single charge pump voltage is compared to a supply voltage or some process dependent voltage such as V.sub.T in a MOS. In the differential case, an output of a phase-frequency detector is a series of "up" and "down" pulses which are used to control a differential charge pump. These up and down pulses are proportional to the phase difference of the inputs to the detector. When connected to a differential charge pump, the result is a voltage difference which is also proportional to the phase difference of the inputs. This voltage difference is applied to a differential voltage to current converter, the output of which is applied to a current controlled oscillator.
A "pseudo-differential" scheme may be used where the input voltage is compared to a locally generated voltage reference. The differential and pseudo-differential schemes allow the current to be steered such that the minimum current is zero and the maximum current is equal to .alpha..sub.2 (the frequency/current gain factor within the current controlled oscillator) times a well-controlled constant bias current.
To ensure loop stability across a wide range, to minimize jitter, and to control frequency acquisition times, it is often desirable to have the gain constant of the phase locked loop programmable. This is commonly accomplished by altering device sizes within the current controlled oscillator, by switching in delay elements within the current controlled oscillator, or by changing current mirror ratios within the voltage to current converter. The gain of the phase locked loop may be controlled by altering the bias current, the voltage to current gain constant and the current controlled oscillator gain constant. However this also affects the frequency of the oscillator. The maximum operating frequency is set by the gain constants and the minimum frequency is equal to zero.